The present invention relates to a semiconductor laser element.
Conventionally, there has been a semiconductor laser element as shown in FIG. 10 (refer to Japanese Patent Laid-Open Publication No. HEI 9-199790). This semiconductor laser element is a semiconductor laser element of an effective refractive index waveguide type. On an n-type GaAs substrate 501, there are provided an n-type GaAs buffer layer 502, an n-type AlGaAs first clad layer 503, a quantum well active layer 504, a p-type AlGaAs second clad layer 505, a p-type GaAs etching stop layer 506, a p-type AlGaAs third clad layer 507 and a p-type GaAs cap layer 508. The p-type AlGaAs third clad layer 507 and the p-type GaAs cap layer 508 are formed in a ridge-like shape to constitute a ridge portion 513. An n-type AlGaAs current light confining layer 509, an n-type GaAs current confining layer 510 and a p-type GaAs flattening layer 511 are laminated on both sides in the widthwise direction of this ridge portion 513 and on the etching stop layer 506. A p-type GaAs contact layer 512 is formed on the cap layer 508, end surfaces of the current light confining layer 509 and the current confining layer 510 and the flattening layer 511. A p-type electrode 514 is arranged on the p-type contact layer, and an n-type electrode 515 is arranged on the lower surface of the n-type GaAs substrate. This semiconductor laser element is mounted on a package and employed as a light source for an optical disk device.
The above-mentioned semiconductor laser element is fabricated as follows. That is, as shown in FIG. 11A, the n-type GaAs buffer layer 502, the n-type first clad layer 503, the non-doped Multiple Quantum Well (MQW) active layer 504, the p-type second clad layer 505, the p-type GaAs etching stop layer 506, the p-type third clad layer 507 and the p-type GaAs cap layer 508 are successively epitaxially grown on the n-type GaAs substrate 501 by a first-time metal-organic chemical vapor deposition method (hereinafter referred to as a MOCVD method).
Next, a stripe-shaped resist mask directed in the [011] direction is formed on the cap layer 508. Part of the cap layer 508 and part of the third clad layer 507 are etched to the etching stop layer 506, forming a ridge portion 513, which has a width of 2.5 μm and extends in the [011] direction (FIG. 11B).
After the resist mask on the cap layer 508 is removed, the n-type AlGaAs current light confining layer 509, the n-type GaAs current confining layer 510 and the p-type GaAs flattening layer 511 are successively laminated on the ridge portion 513 and the etching stop layer 506 by a second-time MOCVD method (FIG. 11C).
A resist mask is arranged on both sides in the widthwise direction of the flattening layer 511, and a portion, which belongs to the current light confining layer 509, the current confining layer 510 and the GaAs flattening layer 511 and is located above the ridge portion 513, is removed by etching (FIG. 11D).
The resist mask on the flattening layer 511 is removed. By a third-time MOCVD method, a p-type GaAs contact layer 512 is formed on the cap layer 508, the end surfaces of the current light confining layer 509 and the current confining layer 510 and on the flattening layer 511 (FIG. 11E).
After arranging the p-type electrode 514 on the surface of the contact layer 512 and arranging the n-type electrode 515 on the lower surface of the substrate 501, cleavage is carried out so that the direction perpendicular to the plane of the sheet of FIG. 11E becomes the direction of a preset resonator length, completing a semiconductor laser element.
If a forward bias voltage is applied to the above-mentioned semiconductor laser element, then a current flows inside the ridge portion 513, and a carrier is injected into a center portion in the widthwise direction of the quantum well active layer 504 corresponding to this ridge portion 513, causing laser oscillation. At this time, a reverse bias voltage is applied to an interface between the current light confining layer 509 and the etching stop layer 506 outside the ridge portion 513, and therefore, almost no current flows outside the ridge portion 513.
According to the above-mentioned semiconductor laser element, the etching stop layer 506 is made to be hardly oxidized by forming the etching stop layer 506 on the p-type GaAs second clad layer 505 of p-type GaAs that has an Al composition ratio smaller than that of the second clad layer 505. The current light confining layer 509 is formed by growing AlGaAs of a high-quality crystal on this etching stop layer 506. With this arrangement, photoabsorption and a leak current in the current light confining layer 509 are restrained to make satisfactory the laser oscillation characteristic of the semiconductor laser element. As described above, the semiconductor laser element effects high-output pulse oscillation so as to be used as the light source of an optical disk device that has a high writing speed.
However, the above-mentioned semiconductor laser element has the problem that the rise time and the fall time of the output become comparatively long and the pulse waveform becomes dull when the pulse oscillation is effected with a high output. This dullness of the pulse waveform deteriorates the quality of a signal to be written in an optical disk, causing a read error in reading the signal written in the optical disk. This is ascribed to the following factors.
That is, the speed of rise and fall of the output during the pulse oscillation of a semiconductor laser element as shown in FIG. 10, i.e., the pulse response speed is defined by the internal resistance of the ridge portion 513 and the capacitance of the outside of the ridge portion 513. If the value of product of the resistance value and the capacitance value is reduced, then the above-mentioned response speed is increased. The internal resistance of the ridge portion 513 can be reduced by increasing the carrier density in the third clad layer 507. Moreover, the capacitance of the outside of the ridge portion 513 can be reduced by expanding the width of the depletion layer generated in the interface between the current light confining layer 509 and the etching stop layer 506 when a bias voltage is applied.
FIGS. 12A and 12B are energy band diagrams of the current light confining layer 509 outside the ridge portion 513, the etching stop layer 506 and the second clad layer 505. FIG. 12A shows the energy band diagram when no bias voltage is applied, while FIG. 12B shows the energy band diagram when a bias voltage is applied. The energy bandgap of the etching stop layer 506 is smaller than the energy bandgap of the second clad layer 505. Therefore, when the bias voltage is zero as shown in FIG. 12A, carriers (holes) are accumulated in the etching stop layer 506. On the other hand, if the bias voltage is applied, a depletion layer is formed from the interface between the etching stop layer 506 and the current light confining layer 509. However, since the holes accumulated in the p-type etching stop layer 506 are not extracted, the depletion layer scarcely spreads to the p-type second clad layer 505 as shown in FIG. 12B, and the width of the depletion layer is narrowed. As a result, the capacitance of the outside of the ridge portion 513 increases to slow down the response speed. Consequently, it takes much time for the rise and fall of the output during the pulse oscillation, and the pulse waveform becomes dull. This reduces the quality of the write signal of the optical disk device that employs this semiconductor laser element.